Motor driven actuators are used to provide a variety of movement or operating functions in many different devices in common use today. For example, actuators are used to operate dampers and valves in HVAC systems, fluid values in manufacturing operations, and door openers. Since the time and duration of each actuation event depends on the specific application, electronically based controllers are used to provide operating power to the actuator at preprogrammed times or upon occurrence of preselected events which sensors attached to the controller detect. Actuators have either linear or rotary output, and the invention to be described may be implemented in either design. The intended use of the invention is in rotary actuators however, and the following discussion is directed to such an embodiment. One should understand that rotary actuator principles can frequently be applied to linear actuators. Further, by the use of a mechanical linkage or a rack and pinion gear set, it is easy to convert rotary movement to linear.
The basic design of rotary actuators is very simple. An electric motor drives the output element through a step-down gear train which amplifies the torque from the motor while reducing the speed of rotation. In the case of a rotary actuator, the linkage is almost invariably a gear train, and in the case of a linear actuator, will usually be a gear train terminating in a rack and pinion for the linear operation. The output speed of a typical rotary actuator is on the order of a few RPM at most, and may be less than one RPM. The maximum range of rotation for a rotary actuator is typically less than one revolution, with the direction of rotation controllable within that range. There are also certain types of non-reversible rotary actuators which rotate a fraction of a revolution at each actuation and are used for driving loads for which such operation is suitable. For example, cam-operated valves can be opened or closed with each half turn of the actuator shaft.
In many of these actuator applications, there is no need for high precision in positioning the load. For example, where the application is partially opening a valve, there already is some uncertainty in the flow rate for a given percentage of valve opening, so high precision in the angular position of the actuator is not necessary. In such uses, the expense of closed loop operation is unjustifiable, but there is a need for accuracy of a few percent or so in achieving a specified position. Use of a synchronous fixed speed motor as the prime mover in an actuator results in output speed which is independent of load or voltage, frequently allowing satisfactory control of actuator position based solely on operating time. A disadvantage of a synchronous motor as the prime mover in an actuator is its relatively high initial cost for a given power output as compared to that of certain types of non-synchronous motors, particularly DC motors. This extra cost reduces the advantage of operating open loop which the synchronous motors have over non-synchronous motors. DC motors are also more efficient than synchronous motors in converting electrical energy into mechanical energy, which may also have significance in certain applications.
In certain actuator applications it is necessary for reasons of safety or convenience to assure that the actuator returns its output element and the load to a particular safety or home position if a power outage occurs or if improper operation of the controller removes power from the actuator. These automatic return types of actuators must store energy in some way in order to provide the power return to home. There are two ways in which the power is currently stored. In some actuators a storage battery provides electrical power used by the motor when the power return function is required. This design has the disadvantages that the battery may deteriorate over time, a separate charging circuit is needed, and weight and cost are substantial.
The more common design of power return actuator has a coil spring which stores energy for release when the return function is needed. In the simplest of these, the spring is permanently connected to the drive train and winds and unwinds as the motor drives the output shaft in the outward and return directions. Such a design has the advantage of simplicity, but there are a number of disadvantages which arise. One is that the torque output is asymmetrical, being substantially less during the outward rotation against the spring than on return. This asymmetry is exacerbated by the well-known characteristic of coil springs to require several times more torque to wind than is provided during their unwinding, mainly because of the friction between adjacent turns of the spring. Further, many loads have symmetric torque requirements, and application of excessive torque in either direction has the potential of damaging the load. For example, if the load is a valve, one will realize from personal experience at a bathroom sink that a valve requires roughly the same maximum torque when opening as when closing. One will also realize that if the closing torque is substantially greater than the opening torque available, it is possible that the actuator will not even be able to open the valve. Of course the problem of asymmetric torque output can be alleviated by use of a slip clutch with a symmetric release torque. But the asymmetric torque level still requires a drive motor large enough to handle the largest power demand by the intended load, increasing cost and power requirements for the actuator itself.
The problem of the relatively high cost to output power ratio for synchronous motors is compounded in direct coupled spring return types of actuators, since it is necessary to specify a synchronous motor for with much more power than normal operation of the load requires, in order to have sufficient additional power to wind the spring during the outward excursion of the shaft. Because of this substantially greater torque requirement, spring return actuators with constant speed during normal operation are relatively large and heavy, use power inefficiently, and are costly.
Because of these disadvantages of direct coupled return springs, a recent design described in U.S. Pat. No. 5,182,498 entitled Spring Return Rotary Actuator and having a common patentee and assignee with the present application solves the problem by introducing a special spring winding phase of operation in which a non-synchronous motor provides helping torque for winding the spring. When power is applied to the actuator, a sensor determines whether the spring is in a fully wound condition. If not, the spring winding phase is initiated during which the spring is fully wound and then held in this condition by a brake. With the spring held locked, operation of the actuator then proceeds until the condition for spring return arises, as though the actuator is a non-return type. When spring return is required, then the brake is released and the spring applies torque through the gear train to the output shaft to return the load to its home position. The actuator described in the '498 patent uses a planetary gear system to connect the load, spring, and motor.
In order for the '498 patent's actuator to reach, during spring-powered operation, the torque and power output which is provided by the motor, it is necessary because of the characteristics of planetary gear systems to use a spring whose direct torque output is roughly equivalent to that of the motor itself. Because of the additional torque required to wind the spring, this design also has required an oversized synchronous motor for power. Various attempts to increase the gear ratio of the synchronous motor when winding the spring so as to require no more motor torque than does normal operation have turned out to be less satisfactory than a motor of adequate size.
Accordingly, any simple and cheap design which might allow the normal torque output level of the synchronous motor in the '498 patent's actuator to wind up the spring has the potential to reduce its cost. However, such designs have not been available to date.